In optical communication systems, optical waveguides provide a transmission channel for guiding an optical signal produced by a light source, e.g., a laser, at one end of the system to a detector, e.g., a photodetector, at the other end of the system. The photodetector material, an active region, absorbs energy from the photons of the transmitted optical signal, which, in response, excites charge carriers, e.g., electrons and holes. With the application of a reverse bias voltage, the excited charge carriers are attracted to contacts on the photodetector, thereby creating an electrical current that corresponds to the optical signal. In this manner, the photodetector converts an optical signal into an electrical signal.
Many optical communication systems utilize long-wavelength optical signals, e.g., 1310 nanometers (nm). Because silicon does not respond to long-wavelength signals, other materials, e.g., germanium, need to be added to the photodetector. For example, due to its potential for being grown on top of silicon, germanium is an appropriate choice for a photodetector if a monolithically integrated photodetector and silicon-on-insulator (“SOI”) photonic device is needed.
The lattice constant refers to the distance between unit cells in a crystal lattice. The lattice constant of the germanium is not perfectly matched with the lattice constant of silicon; the lattice constant of germanium is slightly larger than that of silicon. The mismatch between the lattice constants of germanium and silicon presents problems for using regular epitaxial growth (“EPI”) technique for growing crystals. Currently, two main methods have been heavily studied to make single crystal germanium film on top of silicon substrates: 1) using a buffer layer and post-process after selective epitaxial growth (“SEG”), and 2) using the rapid melt growth (“RMG”) technique. Between these two methods, RMG has better process compatibility but has a limitation on the structures that can be constructed.
In the buffer layer technique, a thin layer of amorphous germanium is deposited onto the silicon. Although the germanium layer created using the buffer layer technique may be thicker than the layer created using other techniques, the resulting germanium layer has defects because the initial crystal layer was not initially perfect. Defects in the photodetector are undesirable because the defects function as impurities inside the crystal materials that can generate free carriers and cause leakage current even when no light is present. The leakage current may cause noise and false signals.
In the RMG technique, germanium is not grown directly on top of the silicon. Instead, poly-germanium is deposited and then a silicon-dioxide coating is applied that surrounds the poly-germanium. The main issue with using RMG to make a waveguide photodetector stems from the nature of the RMG method itself. The RMG method requires a micro-furnace formed by the silicon-dioxide coating surrounding the deposited poly-germanium. Silicon-dioxide is a low index material, which makes it difficult to couple the light into the resulting high index single crystal germanium. A significant amount of photons are refracted due to the difference in the two indices, resulting in energy not being coupled to the photodetector. The coupling problem can be seen in prior efforts that use the RMG method to integrate germanium with silicon for optical devices, e.g., FIG. 1F in U.S. Pat. No. 7,418,166.